A power module (or power semiconductor device) may have a vertical semiconductor element such as an IGBT (Insulated Gate Bipolar Transistor) and a diode mounted thereon as a switching element or a rectifying element. This vertical semiconductor element has, for example, a rear-surface metallized layer (rear-surface electrode) disposed on an entire rear surface thereof and a front-surface metallized layer (front-surface electrode) disposed on a portion of a front surface opposite to the rear surface. Ni, Mo, or Cr may be used as a metal material constituting the metallized layers. The rear-surface electrode is electrically connected to a substrate electrode, and the front-surface electrode is electrically connected through a wiring member to an external terminal, so that a wiring structure is formed for applying a large current to the power module.
To reduce a power loss, semiconductor elements are recently developed by using wide band gap semiconductor materials such as silicon carbide (SiC) and gallium nitride (GaN) instead of silicon (Si). A semiconductor element using a wide band gap semiconductor material has high heat resistance and therefore can operate at a high temperature with a large current and, on the other hand, it is necessary to use a bonding material having high heat resistance performance for achieving the characteristics. However, no lead-free solder material having high heat resistance performance is currently found.
On the other hand, it is strongly required to miniaturize a semiconductor element by improving a heat dissipation property of a bonding portion between a rear-surface electrode and a substrate electrode of the semiconductor element. To improve the heat dissipation property, it is effective to make a bonding layer thinner. A solder material is conventionally used for bonding between the rear-surface electrode and the substrate electrode. However, since the solder material has difficulty in ensuring the reliability of the bonding portion and tends to deteriorate the heat dissipation property due to generation of voids, it is not preferable to make the bonding layer thinner. Therefore, to achieve miniaturization etc. of the power module, needs exist for a bonding material or a bonding method excellent in the heat dissipation property and less likely to cause deterioration in the bonding portion.
Thus, studies are conducted for a power module having a semiconductor element bonded to a substrate by using a sinterable metal bonding material utilizing a sintering phenomenon of metal fine particles, instead of the solder material (see, e.g., Patent Documents 1 to 3). The sinterable metal bonding material is a pasty bonding material composed of metal fine particles, an organic solvent component, and a protective film covering the metal fine particles. The sinterable metal bonding material achieves metal bonding to a member to be bonded by utilizing a phenomenon that the metal fine particles are sintered at a temperature lower than the melting point of the metal. After bonding, diffusion bonding occurs between the metal fine particles, and diffusion bonding occurs also between a metalized layer of a semiconductor element and a surface of a substrate on which the semiconductor element is mounted, so that the melting point after bonding is increased to the original melting point for the metal. As a result, the power module using the sinterable metal bonding material may have a heat resistance performance higher than the temperature at the time of bonding. Additionally, gold (Au), silver (Ag), and copper (Cu) generally well known as the sinterable metal bonding materials have a thermal conductivity greater than the solder material, can make a bonding layer thinner, and therefore has a high heat radiation performance.
As described above, the sinterable metal bonding material has high heat resistance performance. However, using the sinterable metal bonding material causes the following new problem that does not occur when the solder material is used. Specifically, since a stress is released in the solder material due to development of cracks inside, stress concentration hardly occurs on a base and a metallized layer of a semiconductor element. On the other hand, the sinterable metal bonding material has high strength and therefore has a problem that when thermal stress is repeatedly applied, a damage is caused in the base and the metallized layer of the semiconductor element due to occurrence of cracks etc. in the bonding portion, which makes the reliability of the power module liable to deteriorate.
To deal with this problem, for example, as shown in FIG. 8 of Patent Document 3, it is conceivable that a low thermal expansion plate is disposed between the semiconductor element and the substrate electrode (base electrode) to increase the bonding layer as a whole and thereby relax the thermal stress applied to the bonding portion of the semiconductor element so as to improve the reliability of the power module.